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Copper.                                       83

in toughness, and therefore inferior to wrought iron in that
respect. It may be improved by the annealing spoken of, but
steel that is required for boiler or bridge work must be capable
of resisting vibration, and so a milder quality is used, which,
though it may not be considerably stronger than iron, is more
homogeneous and has a finer grain.

The amount of carbon varies with the use to-which the steel
is to be put, and is shewn by the following table :

Razor tern Sawfile Tool Spindle Chisel Sett Die
	Per ......
	iJj l  Carbon .....
	Will spoil with over- heating. To be heated only cherry red. May weld with great care. Ditto. Tough ; will harden at low heat. Stands hammer ; welds easily. Stands pressure ; welds like iron.

		if/.      ......
 1*7     .,     ......
	i   7
		i'/.        ......
Cutting tools require most carbon.    (See Appendix //.,,/. 793.)

Tempering or the capability of receiving any degree of hard-
ness, is a property of steel, and was formerly applied as a test
to distinguish it from wrought iron; while case-hardening is a
method of partially converting wrought iron into steel, but both
these subjects will be reserved for our next chapter.

Test.A rough test to distinguish between wrought iron
and steel is to put a drop of dilute nitric acid on the
polished metal, when a greenish-grey stain will indicate iron,
and a black spot will shew steel; the purer the black, the
more carbon may be suspected, so that we may even get a notion
of quality.

(5.) Copper Ore is various in character, but may have iron,
sulphur, antimony, or arsenic associated with it. The operations
are three in number:(r) Roasting^ to get rid of arsenic and
sulphur, the iron forming an oxide. (2) Smelting, to dissolve
the iron oxide, and leave copper combined with sulphur. (3)
Roasting and Smelting, to remove the sulphur and obtain metallic
copper. The furnaces used throughout are of the same class as
the puddling furnace, Fig. 85, and called rewrberatory on account
of the arch beating back the flame. Other refining processes have